Sustained global dependence on fossil fuels as a primary source of energy is beset by several intractable problem, including dwindling reserves, increasingly unacceptable levels of pollution and relatively low conversion efficiency. Of several proposed alternative technologies, hydrogen-based fuel cells have emerged as being particularly attractive, especially for mobile applications. Much still needs to be accomplished in order to realize the necessary infrastructure and technological advances that will ultimately lead to the production, purification, transport, storage, and conversion of hydrogen as an everyday consumer commodity.
In the short term, escalated production of hydrogen will most likely continue to rely on conventional technologies such as steam reforming of natural gas:CH4+H2O→CO+3H2 followed by the water gas shift reaction:CO+H2O→CO2+H2 The final step in this process involves the purification of H2 by removing impurities such as CH4, CO, H2O, and primarily CO2.
Separation technology is critical to the deployment of hydrogen as a source of energy, since the purity of hydrogen supplied to fuel cells affects their performance and longevity, and therefore their economic viability. To fulfill its promise, the hydrogen economy will require compact, durable, and inexpensive purification devices.
Conventional hydrogen plants are generally based on the use of pressure swing adsorption (PSA) for final hydrogen purification. PSA utilizes the difference in adsorption properties of various molecules so that components of a gas mixture are selectively adsorbed onto a solid matrix at high pressure and then subsequently desorbed by lowering the pressure. In recent years, both design and operation of PSA processes have developed to the extent that any notable further improvements in gas separation necessitates the discovery of a novel adsorbent material. Zeolites and activated carbon are currently employed as the solid matrix. While carbon nanotubes and metal-organic frameworks have undergone substantial scrutiny in this area, molecular crystals have received little consideration. This is likely because the constituent molecules of molecular crystals generally pack with an efficiency that has been deemed to preclude porosity.
Calixarenes are complex cyclic compounds that can undergo self-assembly to form supramolecular crystalline complexes. The simplest calixarenes is calix(4)arene in which four phenyl groups are linked together in a cyclic array by methylene bridges that are proximal to the OH groups of the phenols. Stress induced within the calixarene molecules result in indented or bowl-shaped cavities that can result in lattice voids. However, calixarenes lattices are considered to be non-porous and do not have any channels providing access to these lattice voids.
The assembly of organic molecular crystals such as those based on supramolecular assemblies of calixarene molecules is primarily controlled by a variety of intermolecular interactions that, in unison, immobilize the building blocks to form stable arrays. When these materials are heated beyond their melting or sublimation points, the cohesive forces are overcome, resulting in increased mobility and disorganization of the molecules. The molecules of a solid can also be mobilized by processes such as dissolution and solid-solid phase changes. The latter can occur as a result of physical stimuli (e.g. temperature, pressure, radiation) or the gain or loss of ancillary molecular components.
While inclusion of either a liquid or a gaseous guest by a solid matrix is a well-known phenomenon, the mechanisms of such processes are not well defined. In organic solid-state guest-host assemblies, transport of the guest through the host, and subsequent complexation, usually involves concomitant reorganization of the host lattice. Guest-induced lattice rearrangement often result in severe fracturing of single crystals into polycrystalline material. When fracturing does not occur, alternative mechanisms postulate the presence of stable channels through which mobile guest molecules diffuse until a thermodynamically stable host-guest structure is achieved. Therefore, it would not appear that relatively small volatile gas molecules, such as N2, O2, air, CO, and CO2 that do not have strong intermolecular interactions to provide the impetus for lattice rearrangement, would be able to be incorporated into a nonporous crystalline lattice.